1. Field of the Invention
The present invention relates to a gas sensor for detecting a concentration of a specific gas contained in a gas under measurement, and a method of manufacturing the gas sensor.
2. Description of Related Art
There is known a gas sensor for detecting a concentration of a specific gas contained in a gas under measurement, the gas sensor including a sensor element having a structure in which a sensor cell made of an oxygen ion-conductive solid electrolyte body provided with sensor electrodes at its both surfaces, and a heater including a heat generating section to generate heat for heating the sensor cell are formed integrally with each other. For example, refer to EP Patent Application Publication No. 0506897. This sensor element includes two pairs of electrode pads respectively connected to the heat generating section and the sensor electrodes. These electrode pads of the two pairs are formed at a surface of a portion of the sensor element which is closer to the proximal end of the sensor element than a portion at which the sensor element is insert-held by an insert-holding insulator. These electrode pads are formed such that two of them are arranged side by side on one of the two electrode forming surfaces parallel to each other of the sensor element, and the other two of them are arranged side by side on the other of the two electrode forming surfaces of the sensor element.
In more detail, as shown in FIG. 31, metal terminals 932 respectively connected to external leads are disposed in contact with their respective metal electrode pads 921. The metal terminals 932 are four in number and disposed such that two of them are arranged on the inner surface of one of proximal end insulators 931 of one pair disposed to hold a proximal end portion 922 of the sensor element 92 at electrode forming surfaces 923 of the proximal end portion 922, and the other two of them are arranged on the inner surface of the other of the proximal end insulators 931. The proximal end insulators 931 are pressed in the direction approaching each other by a spring member 933 of a ring shape disposed surrounding the proximal end side insulators 931. This keeps the two pairs of the metal terminals 932 in a state of being abutted against the two pairs of the electrode pads 921, respectively.
However, since the above gas sensor uses the proximal end insulators separated from each other, it has a problem in that the metal terminals 932 tends to move relative to the electrode forming surfaces of the sensor element when the metal terminals are connected to their leads, or when a bush is installed in the vicinity of the proximal end portion of the gas sensor during assembly of the gas sensor. The contact pressure of the metal terminal to the electrode pad may be reduced significantly especially when the metal terminal is applied with a force which causes the metal terminal to move away from the sensor element.
The variation in the contact pressure between the metal terminal and the electrode pad causes variation in the contact electrical resistance therebetween, as a result of which the detection accuracy of the sensor cell may be lowered, and the activation time of the sensor cell may be prolonged.